1. Field of the Invention
The present invention relates to image processing of a full color image.
2. Description of Prior Art
A copying machine can support electronic sorting and background control. In the electronic sorting, image data of a plurality of pages of a document is stored temporarily in a memory device, and the stored image data is read in a predetermined order for printing to sort the printed papers. Because the storage capacity of the image data is large, the image data is compressed before stored in the memory device, and the compressed image data are expanded before printing. In the background control, the color of background is set to white for a character image.
Both image data compression and background control may be considered for printing. For example, in Japanese Patent laid open Publication 2001-16452, the underground removal is taken into account when nonreversible compression and expansion are performed with a Joint Photographic Expert Group (JPEG) algorithm. In order to remove mosquito noises liable to occur near an edge in an image, the background is removed twice before and after the compression and the expansion, without deteriorating the gradation characteristic of the image data.
However, for a full color image, the electronic sorting is not supported practically because it needs a large storage capacity of a memory device. Because a memory device having such a large storage capacity becomes available recently, it is under review to equip the electronic sorting function in a full color copying machine or the like. When the electronic sorting function is equipped, it is desirable to increase compression ratio of the image data to be stored in a memory device. If the background control function is also equipped, the correlation of the compression ratio of the image data with the background control has to be considered. The background of an image may be controlled in two types. In the automatic background control, the background control level is set according to the image data, while in the automatic control function, a fixed background control level is set manually. If the background control level is fixed, the compression ratio becomes higher. Therefore, in the image processing in a copying machine or the like, it is a problem where the automatic manual background control functions and where the memory device for electronic sorting is positioned.